This invention relates generally to rotor winding shorting on a wound rotor type induction machine and, more particularly, to methods and apparatus employing electronic circuits to short-circuit the rotor winding.
In a wound rotor motor, to provide a balanced simple winding which will result in low reactance and good performance characteristics, the rotor slots will be multiples of the poles and the phases. With the stator arranged the same way, the result gives rise to permeance locking torques at standstill or zero motor speed. Typically, to initially rotate the rotor, a high resistance is inserted into a rotor circuit to produce torque and limit current. As the angular speed of the rotor increases, the resistance is decreased. Typically, the external rotor resistance circuit is electrically connected to the rotor winding via collector (slip) rings and brushes.
As the speed approaches rated values, the windings are shorted so that a sufficient magnetic field can be induced into the rotor windings from the stator winding to produce the required torque. However, supplying a short circuit to the rotor windings through the collector rings and brushes is inefficient because of the brush wear caused by a friction between the rings and brushes. Additionally, since most brushes are carbon based, carbon dust typically accumulates in the motor from the brush wear.
Originally, shorting the collector rings was done by manual operations, such as, for example, a knife switch across the collector rings supply circuit shorts across the rotor windings, and then the brushes are manually lifted from the rings. This type of solution is not desirable, for safety purposes, when applied to high voltage, high horsepower machines.
Other methods use a motor driven plate with shorting studs which were moved into place to short the collector rings using a worm gear and an electrically driven brush lifting gear, or alternatively a plate with shorting studs is hydraulically driven into place to short the slip rings.
It is desirable to employ a method of shorting out the windings of the rotor which did not employ moving parts, thereby, enhancing the safety and reliability of such an operation. It is also desirable that such a method operates independent of the slip rings and brushes used in known machines.
The present invention, in one aspect, is a method for shorting rotor windings in a wound rotor induction machine, the method including monitoring rotor current for a frequency indicative of a desired steady-state operating condition and electronically shorting the rotor windings when the monitored rotor current frequency reaches a defined threshold indicative of the desired steady-state operating condition.